1. Field of the Invention
The present invention relates to a heat-treatment apparatus for performing a heat treatment of semiconductor wafers and, more particularly, to an exhaust system for removing exhaust gases from a heat-treatment furnace.
2. Description of the Related Art
Recently, in the heat treatment process of manufacturing semiconductor devices, the size of a semiconductor wafer to be treated is increased from six inches (four inches) to eight inches, and the number of wafers to be subjected to batch processing in a heat-treatment furnace is also increased from 100 pieces to 150 pieces. For this reason, the type of a heat-treatment furnace to be used is shifted from a horizontal type to a vertical type, resulting in a great increase in furnace capacity.
In such a large, vertical type heat-treatment apparatus, since large amounts of exhaust gases are generated in oxidation and diffusion processes, an exhaust system is arranged to exhaust them out of the process system.
Published Unexamined Japanese Patent Application Nos. 62-63421, 63-238281, and 63-304620 respectively disclose exhaust systems having flow control valves. In these exhaust systems, an exhaust apparatus is complicated and expensive. In addition, a CVD apparatus, in which reaction products tend to adhere to the exhaust system, requires frequent maintenance and inspection.
Published Unexamined Japanese Patent Application No. 58-124226 discloses an exhaust system in which the pressure of exhaust gases from a process tube is kept constant in relation to a pressure corresponding to a liquid column. In this exhaust system, only when the internal pressure of an apparatus becomes larger than the atmospheric pressure, exhaust gases in the process tube are exhausted. In this system, however, if the internal pressure of the process tube becomes smaller than the atmospheric pressure, no exhaust gases are exhausted, and the flows of gases in the process tube become nonuniform. As a result, a film having a uniform thickness cannot be formed on a wafer surface.
As shown in FIG. 1, a collective exhaust apparatus 7 is generally installed in a semiconductor device manufacturing factory so as to perform an exhausting operation of the overall factory. This collective exhaust apparatus 7 includes a high-power fan 8. In addition, exhaust paths 5b of a plurality of heat-treatment apparatuses 2 (only one apparatus is shown in FIG. 1 but the other apparatuses 2 are omitted) merge into a path 5c of the collective exhaust apparatus 7. That is, the plurality of heat-treatment apparatuses 2 share the single exhaust system 7. Referring to FIG. 1, reference numeral 3 denotes a gas reservoir; 3a, a mass flow controller; 4, combustion apparatus (Steam generator); 5, a process tube; 5a, 5b, and 5c, gas pipes; and 6, a heater. Reference symbol B denotes a wafer boat; and W, a semiconductor wafer.
In such a collective exhaust apparatus 7, when all the heat-treatment apparatuses 2 in the factory are fully operated, exhaust gases in the process tubes 5 can be properly and uniformly exhausted. If, however, only some of the heat-treatment apparatuses are to be operated, the pressure in each process tube 5 is difficult to control to be a desired value. More specifically, the internal pressure of the exhaust path 5c of each heat-treatment apparatus 2 becomes an excessively large negative value, and the pressure in the process tube 5 tends to be smaller than that in a normal state. For this reason, the internal pressure of the process tube 5 does not reach a predetermined value, and a film having a desired thickness cannot be formed on a surface of a semiconductor wafer W.
In addition, in the collective exhaust apparatus 7, its exhaust power varies due to various factors such as voltage variations, and strong and weak exhaust flows are produced, resulting in so called pulsating flows. Pulsating flows in such a collective exhaust system disturb a gas flow in the process tube 5 and adversely affect the manufacture of semiconductor devices, such as variations in thickness of films formed on the wafers W.
Published Unexamined Japanese Patent Application No. 2-59002 discloses a trap unit for trapping reaction products contained in exhaust gases in a pipe of an exhaust system.
Published Unexamined Japanese Patent Application No. 61-160933 discloses a waste liquid processing system of a semiconductor substrate developing apparatus.
In the above-described exhaust system, however, an exhaust gas is cooled while it flows in the system, and a gas component whose temperature is lowered below a condensation temperature is condensed on a pipe wall. As a result, a waste liquid collects in the pipe.
In an apparatus used for a special treatment among vertical type heat-treatment apparatuses, since a waste liquid is produced in a pipe in large quantities which cannot be ignored, various problems are posed. Especially, in a technique of forming an oxide film by oxidizing a semiconductor wafer surface, since steam or humidified oxygen gas (wet O.sub.2 gas) is used as a process gas, steam in the process gas is condensed on a pipe of the exhaust system, and a large amount of waste liquid is produced.
Of such apparatuses, especially in a steam oxidation apparatus, the amount of waste liquid is large, and a waste liquid tends to collect in a pipe of an exhaust system. If a large amount of waste liquid collects in the pipe, an exhaust path of an exhaust gas is narrowed by the waste liquid, and an exhaust gas in a process tube cannot be sufficiently exhausted. As a result, a gas flow in the process tube becomes nonuniform, and the thicknesses of films formed on semiconductor wafers subjected to batch processing vary. This impairs quality stability.
In another oxide film forming technique, in order to obtain a high-quality oxide film, a gas mixture obtained by mixing hydrochloric acid in a dry oxygen gas is sometimes used. In an exhaust system for exhausting such a gas, since stainless steel pipes tend to be corroded by hydrochloric acid, the inner surface of the pipe must be periodically cleaned. In cleaning, the pipes must be disassembled, and a time-consuming operation is required, resulting in a decrease in operating efficiency of a heat-treatment apparatus.
Furthermore, in order to remove adhesive contamination in a process tube, an HCl gas is introduced in the process tube while no semiconductor wafers are processed. If such a cleaning gas component is left in a stainless steel pipe of the exhaust system, the pipe is corroded and must be frequently replaced with a new one. This decreases the operating efficiency of the heat-treatment apparatus.